Product

ABSTRACT

Hollow membrane fibers are formed from a mixture of N-alkoxyalkyl polyamide, polyvinyl alcohol, di(lower alkyl) sulfoxide and water. The fibers are useful for separating chemicals, e.g., aliphatically-unsaturated hydrocarbons, from mixtures containing them.

This is a division of application Ser. No. 419,091, filed Nov. 26, 1973now U.S. Pat. No. 3,940,469.

This invention relates to the formation of hollow membrane fibers by theuse of compositions containing N-alkoxyalkyl polyamide and polyvinylalcohol. More specifically, the invention concerns the formation of suchfibers from mixtures containing the N-alkoxyalkyl polyamide, polyvinylalcohol, di(lower alkyl) sulfoxide and water. The fibers areparticularly useful for separating chemicals, e.g.,aliphatically-unsaturated hydrocarbons, from mixtures containing them bythe combined use of liquid barrier permeation and metal-complexingtechniques. A separation of particular interest is that of ethylene fromone or both of methane and ethane, with or without the presence ofhydrogen.

In patent application Ser. No. 395,055, filed Sept. 7, 1973, now U.S.Pat. No. 3,980,605, it is disclosed that hollow fibers can be made byextruding mixtures containing N-alkoxyalkyl polyamide, polyvinyl alcoholand di(lower alkyl) sulfoxide. The fibers are particularly useful inseparating aliphatically-unsaturated hydrocarbons from mixturescontaining them. In these separation procedures, it is preferred thatthe membranes be in the form of hollow fibers since they offer theadvantages of high surface area per unit volume, thin fiber walls forhigh permeation rates, favorable ratios of fiber outside diameter toinside diameter in order to withstand high pressures, and low cost persquare foot per area of membrane surface. Although one could considerforming these fibers by melting together the N-alkoxyalkyl polyamide andpolyvinyl alcohol to provide a mixture of extrusion consistency, thisprocedure is disadvantageous since the polymers start to decomposebefore their melting point is reached. The provision of a di(loweralkyl) sulfoxide such as dimethyl sulfoxide (DMSO), in the mixture leadsto the successful formation of useful fibers by a hot extrusiontechnique, and such preparations are described in the above-identifiedpatent application. The temperatures used in this operation are,however, frequently sufficiently elevated that some undesirablereaction, e.g. decomposition, of the polymers may occur and cause themto become discolored. Also, the polymers may become unduly cross-linkedto make their extrusion difficult, if not impossible, and the polymersmay react with the di(lower alkyl) sulfoxide.

When the mixture of polymers is melted with DMSO undesirabletemperatures in the range of about 120° to 150°polymer-di(lower C. it isgenerally stirred vigorously and the melting way require a considerableperiod of time, e.g. about 2 hours in certain types of equipment. Thepolymers and DMSO may also be held as a melt for a period of time whilethe mixture is in a tank used to supply feed to the extruder. Thegreater the temperature of the mixture or the greater the time it ismaintained at elevated temperatures, the more severe is the heatingoperation and the greater the tendency of the polymers to undergoundesirble reactions. These reactions may be indicated by the mixtureturning brown, and at very severe conditions, e.g. 150° C. for 4 hours,the melt may form a dark brown, tarry mass.

By the present invention, it has been found that the provision of waterin the extrusion mixture containing the N-alkoxyalkyl polyamide,polyvinyl alcohol and di(lower alkyl) sulfoxide inhibits the degradationof the polymerdi(lower alkyl) sulfoxide system and significantly reducesthe likelihood of encountering the problem. By the use of water in theextrusion mixture, a relatively uniform composition can be made withlittle, if any, decomposition of the polymers occurring, before orduring extrusion.

The compositions of the present invention which are used to form thehollow fibers are composed of N-alkoxyalkyl polyamide, polyvinyl alchol,di(lower alkyl) sulfoxide and water. The compositions are generallycomprised of hydrophilic, fiber-forming amounts of the polyamide andpolyvinyl alcohol, say about 30 to 85, preferably about 40 to 70, weightpercent of N-alkoxyalkyl polyamide; and about 15 to 70, preferably about30 to 60, weight percent polyvinyl alcohol based on the total weight ofthese components. The composites contain sufficient of the di(loweralkyl) sulfoxide to provide an intimate, compatible admixture of thepolyamide and polyvinyl alcohol suitable for forming the membrane. Thesecompositions may often contain about 70 to 400 weight percent of thedi(lower alkyl) sulfoxide, preferably about 90 to 250 weight percent,based on the total weight of the polyamide and polyvinyl alcohol. Thewater in the mixture is generally a sufficient amount to inhibit thedegradation of the mixture of polymers and di(lower alkyl) sulfoxide atelevated temperatures, but the amount is not so large that the mixtureis incompatible or no longer of fiber-forming, e.g. extrusion,consistency. The amount of water present in the composition is oftenabout 1 to 30 weight percent, preferably about 2 to 20 weight percent,based on the total of the water and di(lower alkyl) sulfoxide. If theamount of water is too great difficulties in melting the polymer may beencountered, while if the amount of water is too low problems withregard to degradation of the polymer may result.

The hollow fibers can be made by extrusion of the N-alkoxyalkylpolyamide, polyvinyl alcohol, di(lower alkyl) sulfoxide and watermixture. To facilitate handling, the mixture can be allowed to gel whichmay also decrease any tendency towards separation of the polymer andsolvent components during melting of the mixture. A suitable process forextruding the fibers involves providing the mixture having the polymersin solution at an elevated temperature suitable for extrusion, forinstance, a temperature of about 60° to 125° C., preferably about 70° to110° C. Heating of the compositions under severe temperature-timeconditions, e.g. 150° C. for 4 hours, can lead to reactions whichdiscolor the mixture and may even cause the formation of unusable darkbrown, tarry masses. Excessive degradation should be avoided and weprefer that the compositions not be heated under conditions which causeany significant discoloration. The hot material is extruded to formfibers having a hollow core surrounded by the membrane wall. Duringextrusion it is advantageous to pass a gas through the core of thehollow fibers to help cool the fibers and prevent the core of the fibersfrom closing.

After extrusion the fibers can be dried or otherwise treated to removethe di(lower alkyl) sulfoxide and water. If the di(lower alkyl)sulfoxide is allowed to remain in the fibers in significant quantities,the fibers may be deleteriously affected over a period of time. Forexample, if the di(lower alkyl) sulfoxide, e.g. DMSO, remains in thefiber for extended periods, the fiber may become paste-like inappearance.

If removal of the di(lower alkyl) sulfoxide from the fibers is postponedafter forming, the fibers may be weaker than if the di(lower alkyl)sulfoxide had been withdrawn sooner. Thus we prefer to remove most, ifnot essentially all, of the di(lower alkyl) sulfoxide from the fibersmore or less immediately after they are formed from the solution. Apreferred method of removal is by drying at an elevated temperature,e.g. about 60° to 110° C., with shorter times being usable at highertemperatures.

Other methods for removing di(lower alkyl) sulfoxide from the fibers maybe employed. These techniques include solvent washing procedures inwhich the fibers are contacted at suitable temperatures with a liquidorganic solvent for the di(lower alkyl) sulfoxide. The solvent andtemperatures employed should not unduly dissolve or otherwisedeleteriously affect the fiber, and washing temperatures from belowambient to about 75° C., may suitably be employed. Among the suitableorganic solvents are the oxygen-containing solvents such as the loweraliphatic ketones, e.g. acetone, and lower alkanols, e.g., methanol andisopropanol, and the low molecular weight paraffins or halogenatedparaffins, e.g. the chlorinated paraffins, e.g., chloroform. Thesolvents may be partially miscible with the di(lower alkyl) sulfoxideand water, and do not significantly dissolve, swell or react with thepolymers present.

In one fiber-forming procedure we have employed, the fibers are extrudedinto a quench bath containing one or more of the foregoing describedsolvents, e.g., acetone or isopropanol, and soaked for a period of about15 minutes to 2 hours or more. The temperature of the quench bath maysuitably be from below ambient, e.g. down to about -20° C., to up toabout 75° C. The fibers are then removed from the bath, dried, say at75° C. for 2 hours; or annealed, e.g. at 125° to 200° C. for up to 4minutes, and then dried, for instance, after cross-linking andwater-washing. The hollow fibers have sufficient thickness so as not tobe readily ruptured or otherwise undergo physical deterioration at arate that would make their use unattractive. Generally the thickness ofthe fiber wall may be up to about 30 mils or more, preferably about 0.5to 15 mils, and often the thickness is at least about 0.1 mil. Theoverall diameter of the fiber may usually be up to about 75 or moremils, preferably about 1 to 30 mils.

The physical characteristics of the membranes, e.g. their strength andchemical resistance, may be enhanced by cross-linking the polyamide orthe polyvinyl alcohol. Cross-linking of the polyamide can beaccomplished by contact of the fibers with an organic or inorganicacidic catalyst such as a sulfonic acid of an aromatic hydrocarbon, mildnitric acid and the like. Such catalysts may, for instance, benaphthalene or toluene sulfonic acids, and cross-linking can beaccomplished at elevated temperatures. During contact of the fibers withthe acid catalyst as an aqueous solution, it is preferred that awater-soluble alkali metal salt be dissolved in the solution to maintainthe integrity of the polyvinyl alcohol by reducing its tendency todissolve in the aqueous catalyst solution. Cross-linking or othermodification of the polymer composition may be effected before, duringor after it is formed into the fiber, but if the modification occursbefore fiber-formation it should not be so extensive that thefiber-formation may not be accomplished.

The properties, for instance, the strength and permeability, of themembrane fibers may be improved by drawing or stretching them and thiscan be accomplished at ambient or elevated temperatures. Suitableelevated temperatures include about 90° to 300° C., preferably about125° to 200° C. The fibers may also be annealed at such temperatures,and the stretching and annealing may be accomplished simultaneously. Thedrawn fibers have a reduced overall diameter and thinner walls thanbefore stretching whether at ambient or elevated temperature, and thistreatment may preferably increase the length of the fibers by a factorof at least about 1.25, say up to about 10 or more. The treatment maydecrease the thickness of the walls to where they are less than about0.5 of the thickness they had before stretching. Excessive stretchingmay adversely affect the strength and performance of the fibers and thuswe prefer that their length not be increased by a factor of more thanabout 9.

The stretching of the fibers is preferably accomplished when they areswollen with an aqueous or organic liquid, especially when stretching isconducted essentially at room or ambient temperature. The swelling agentis preferably water, but it may be an organic swelling agent such asthose listed below as swelling agents. The amount of swelling agentpresent during stretching is often a minor amount up to about 50 weightpercent of the fiber, and preferably is at least about 1 weight percent.The presence of the swelling agent may make stretching easier, e.g.require less force or lower temperatures for the same stretch. Theswelling may preserve a place in the polymer structure for thecomplex-forming solution which is later incorporated in the fibers.

Stretching the fibers generally results in an increase in lengthproportional to the square of the reduction in the inter and externaldiameters of the fibers. The rate of stretching and the time the fibersare at elevated temperatures can affect their properties. Reduction indiameter upon stretching the fibers can be a very rapid process. Thusapplying a force of several hundred grams to a fiber at 200° F. for 0.1of a minute may stretch the fiber adequately. Maintaining this force andtemperature over several minutes has not greatly increased the amount ofstretch. The maintenance of the elevated temperature may cause plasticflow within the fiber which tends to heal any voids produced during thestretching process. Thus, in our process we would prefer keeping thefiber at elevated temperatures for a longer time than is required justfor stretching to benefit from the annealing or healing process.

The materials which are employed to make the semipermeable filmmembranes of the present invention, have a film-forming N-alkoxyalkylpolyamide as an essential component. The polyamide film-formingmaterials are generally known and have also been designated as nylons.The polymers are characterized by having a plurality of amide groupsserving as recurring linkages between carbon chains in the productstructure, and the polymers may be made by several procedures. Commonly,the polyamides are formed by reacting a polyamine and a dicarboxylicacid or its derivative such as an ester, especially a lower alkyl esterhaving, for instance, about 1 to 4 carbon atoms in the ester group.Other reactions which may be employed to form the polyamides include theself-condensation of monoamino, monocarboxylic acids and the reactionsof cyclic lactams. In any event, the polyamide products containrecurring amide groups as an integral part of the principal polymerchain. The polyamides are described, for instance, in the Kirk-Othmer,Encyclopedia of Chemical Technology, Second Edition, Volume 16,beginning at page 1, Interscience Publishers, New York, 1968. Among thetypical structural formulas of the linear polyamides are H₂NRNH(COR'CONHRNH)_(n) COR'COOH and H₂ NRCO(NHRCO)_(n) NHRCOOH, where Rand R' represent primarily carbon-to-carbon chains between functionalgroups in the reactants, and n represents the degree of polymerizationor the number of recurring groups in the polymer chain. The polyamideswhich can be used in this invention are generally solid at roomtemperature, and have a molecular weight which makes them suitable forforming the desired membranes, for example, about 8,000 to 20,000.

The carboxylic acids which may be used in forming the polyamides have anacyloxy group (--R--COO--) in their structure and the R member of thisgroup is composed essentially of carbon and hydrogen and often containsabout 6 to 12 carbon atoms. Such groups may be aliphatic, includingcycloaliphatic, aromatic, or a mixed structure of such types, but thegroups are preferably aliphatic and saturated with respect tocarbon-to-carbon linkages. These R groups may preferably have straightchain carbon-to-carbon or normal structures. Among the usefuldicarboxylic acid reactants are adipic acid, sebacic acid, azelaic acid,isophthalic acid, terephthalic acid, and the methyl esters of theseacids.

The polyamines employed in making the polyamides generally have at leasttwo non-tertiary, amino nitrogen atoms. These nitrogen atoms may beprimary or secondary in configuration, although amines having at leasttwo primary nitrogen atoms are preferred. The polyamines may also haveboth primary and secondary nitrogen atoms and the polyamines may containtertiary nitrogen atoms. The preferred polyamine reactants havealiphatic, including cycloaliphatic, structures, and often have from 2to about 12 carbon atoms. Also, the preferred polyamines are saturatedand have straight-chain structures, although branched-chain polyaminescan be used. Among the useful polyamines are ethylene diamine,pentamethylene diamine, hexamethylene diamine, diethylene triamine,decamethylene diamine and their N-alkyl substituted derivatives, forinstance, the lower alkyl derivatives which may have, for instance, 1 to4 carbon atoms in the alkyl substitutents.

The polyamide polymers which are employed in this invention are those inwhich the film-forming polyamide is an N-alkoxyalkyl-substitutedpolyamide. Materials of this type are well known, as shown, forinstance, by U.S. Pat. Nos. 2,430,910 and 2,430,923, which discloseN-alkoxymethyl polyamides made by the reaction of a polyamide polymer,formaldehyde and alcohol. Generally, at least about 5% of the amidegroups of the polymer are substituted with alkoxyalkyl groups and suchsubstitution may be up to about 60% or more. Preferably, thissubstitution is about 10 to 50% with the product being soluble in hotethanol. Advantageously, these polymers themselves are hydrophilic andabsorb at least about 5 weight percent water when immersed in distilledwater for one day at room temperature and pressure.

The alcohols employed in making the N-alkoxyalkyl polyamides aregenerally monohydric and may have, for instance, from 1 to about 18 ormore carbon atoms. The lower alkanols are preferred reactants,especially the lower alkanols having 1 to 4 carbon atoms. Among theuseful alcohols are methanol, propanols, butanols, oleyl alcohol, benzylalcohol, lauryl alcohol and alcohol ethers, for instance, the alkylethers of ethylene glycol.

The N-alkyloxyalkyl polyamides employed in the present invention toprovide the desired semi-permeable membrane may be reacted withcross-linking agents, especially after the fibers are formed. Forinstance, the fibers may be combined with the cross-linking agent andthese materials may react under the influence of heat. The cross-linkingagents may be, for example, polycarboxylic acids, especially thedicarboxylic and tricarboxylic acids which may have, for instance, from2 to about 12 carbon atoms. Useful acids include oxalic acid, citricacid, maleic acid, and the like. The water-soluble, alkali metal saltsof the polycarboxylic acids, e.g. sodium citrate, may be present in thecomposition during formation of the fibers. Upon acidifying thepolymers, the corresponding carboxylic acid is formed and may serve tocross-link the N-alkoxyalkyl polyamide. If a polycarboxylic acidcross-linking agent be present in an extrudable mixture held at anelevated temperature desired for extrusion, the cross-linking reactionmay proceed to an undesirable extent and make the mixturenon-extrudable. Cross-linking may provide membranes with improvedpermeability when the polyamide is swollen with water at the time thecross-linking reaction occurs. Swelling of the membrane may also beaccomplished with organic liquids such as ketones, and monohydric andpolyhydric alcohols, e.g. alkanols, e.g. propanol, butanol and the like,glycols, glycerol, monoalkyl-terminated glycols or glycol ethers, andthe like. A minor amount, say at least about 3 weight percent, of theswelling agent may be in the fibers when cross-linking takes place,preferably this amount is about 5 to 100 weight percent, based on theweight of the fibers.

The polyvinyl alcohols employed in the present invention are essentiallywater-soluble materials, at least in hot water, and many of these arecommercially available. The molecular weights of these polymers areoften at least about 1000, and are commonly in the range of about 10,000to 300,000. Suitable polyvinyl alcohol polymers are described in, forexample, "Water-Soluble Resins", Second Edition, Edited by Robert L.Davidson and Marshall Sittig, pages 109 to 115, Reinhold BookCorporation, New York, New York. The polyvinyl alcohol may becross-linked, especially after the fibers are formed. The presence ofthe cross-linked polyvinyl alcohol may increase the strength of thefibers and increase their resistance to loss of polyvinyl alcohol byleaching during use. The cross-linking agents used may be polycarboxylicacids, preferably those having from 2 to about 12 carbon atoms. Theuseful acids are preferably water-soluble; and among the polycarboxylicacids, the diacids and triacids, and especially the saturated diacids,are preferred. Included among these are the aliphatic polycarboxylicacids, including oxalic acid, citric acid, maleic acid, malonic acid,and the like. The polyvinyl alcohol may also be cross-linked by reactionwith formaldehyde, e.g. by immersing the fibers in an aqueous bathcontaining 10% Na₂ SO₄ and 3% HCHO, at 50° C. for 1 to 3 hours.

In another method of cross-linking the polyvinyl alcohol, the formedhollow fibers may be combined with the cross-linking agent and thecomposite can be subjected to heat treatment to effect cross-linking.The temperatures used during cross-linking should be sufficient toenhance the cross-linking reaction to the desired degree, but not suchas to affect the fibers detrimentally. The amount of cross-linking agentused may depend upon which agent is chosen, the amount and molecularweight of the polyvinyl alcohol present in the mixture, and the degreeof completion of the cross-linking reaction desired. The amount ofcross-linking agent generally used may be from about 1 to about 100weight percent, and preferably from about 5 to about 60 weight percent,based on the weight of the polyvinyl alcohol.

The di(lower akyl) sulfoxides which may be used to form the membranes ofthis invention are essentially liquid at ambient temperatures of about20° to 25° C. Each alkyl group of these materials often has up to about3 carbon atoms and thus these sulfoxides include dimethyl sulfoxide,diethyl sulfoxide, dipropyl sulfoxide and the like. The use of dimethylsulfoxide (DMSO) is preferred in this invention.

The semi-permeable fiber membranes of this invention have excellentstrength, permeability characteristics and other physical propertieswhich make them specially suitable for use to separate chemicals fromvarious mixtures, and in this use the membranes can be in contact withan aqueous liquid barrier solution which contains complex-forming metalcomponents as ions in solution. Such ions may contain transition metalssuch as silver or other precious metals, copper or the like. Thesemi-permeable membranes are essentially impervious to the passage ofliquid but pervious to gases, under the conditions at which themembranes are used. The fiber membranes are sufficiently hydrophilic tohold the liquid barrier solution at least partly, if not essentiallyentirely, within the fiber membrane. Generally, the fibers will absorbat least about 5, preferably at least about 10, weight percent of waterwhen immersed in distilled water for one day at room temperature andpressure.

The process can be employed to separate various chemicals from otheringredients of a feed mixture providing at least one of the componentsof the mixture exhibits a complexing rate with the material to beseparated or transfer rate across the liquid barrier that is greaterthan at least one other dissimilar or different component of thefeedstock. A pressure differential exists across the liquidbarrier-membrane combination with the exit side of the fibers being at alower pressure than the inlet side. The separated component of the feedmixture is removed from the exit side of the membrane, e.g. by a purgeor sweep gas. Quite advantageously, the system can be used to separatealiphatically-unsaturated hydrocarbons from other hydrocarbons which maybe aliphatically-saturated or aliphatically-unsaturated, or fromnon-hydrocarbon materials, including fixed gases such as hydrogen. Thefeed mixture may thus contain one or more paraffins, includingcycloparaffins, mono- or polyolefins, which may be cyclic or acyclic,and acetylenes or alkynes, and the mixture may include aromatics havingsuch aliphatic configurations in a portion of their structure. Often,the feed mixture contains one or more other hydrocarbons having the samenumber of carbon atoms as the unsaturated hydrocarbon to be separated oronly a one carbon atom difference. Among the materials which may beseparated according to this invention are those having 2 to about 8,preferably 2 to 4 carbon atoms such as ethylene, propylene, butenes,butadiene, isoprene, acetylene and the like. These separation proceduresare described further in the foregoing cited patent application which isherein incorporated by reference.

The following examples will serve to illustrate the present invention.

EXAMPLE 1

In this example extrusion of the polymer mixture was conducted under anitrogen pressure between 200 and 1000 psi on a feed tank and with theextruder having a heated head. Hollow fibers of the polymer blend wereformed by extrusion through a die having an opening in its center.During extrusion, air or nitrogen was blown through the center of thefiber by passage through a hypodermic needle extending into the openingin the middle of the die. After extrusion the fibers were stretchedunder their own weight by allowing the fibers to drop below the extruderhead. The stretched fibers were crosslinked by immersion in a 3%p-toluene sulfonic acid in 10% aqueous sodium sulfate bath for 60minutes at 55° C. The fibers were then washed repeatedly with water toremove the salt from them and allowed to dry.

Fibers were made while using two different extruder heads. One head(Head 1) had a hole 0.067 inch in diameter with a 0.031 inch O.D. needlein the center of the hole, while the other head (Head 2) had a hole0.040 inch in diameter having a 0.020 inch O.D. needle in the center ofthe hole. During extrusion the polymer compositions are forced throughthe annular space between the opening in the extruder head and theneedle, and air is forced through the needle to keep the extruded hollowfiber from collapsing.

A number of different fibers were extruded using various headtemperatures, driving pressures, air flow rates and fiber fallingdistances for drawing. The compositions employed were formed by mixingthe named ingredients at ambient temperature and then raising thetemperature of the mixture to approximately 260° F. to effect melting.The mixture is heated at this temperature for at least 1 hour whileundergoing vigorous stirring before extrusion. The compositionscontained the following:

                  Table I                                                         ______________________________________                                                       Composition                                                    Component           1          2                                              ______________________________________                                        Polyvinyl alcohol (0 to                                                                         80 grams   40 grams                                          0.5 % acetate)                                                               Nylon, Belding    120 grams  160 grams                                        BCI-819, an                                                                   N-methoxy-                                                                    methyl 6:6                                                                    nylon                                                                         DMSO              200 ml.    200 ml.                                          H.sub.2 O         20 ml.     10 ml.                                           ______________________________________                                    

The polyvinyl alcohol employed was Borden's high molecular weight (0 to0.5% acetate) grade, and was determined to have a number averagemolecular weight of about 12,360 by gel permeation chromatography. A 4%aqueous solution of the polymer was determined to have a viscosity of 60centipoises at 20° C. by the capillary tube technique, which mayindicate a molecular weight in the 200,000 range.

The results of these extrusions are given in Tables II and III. Hollowfibers up to 50 feet long were made and were free of holes in the fiberwall and free of plugs in the fiber bore.

                                      Table II                                    __________________________________________________________________________    Extruder Head 1 - Composition 1                                                    T° F.                                                                          T° F.                                                                       Feed Tank                                                   Batch                                                                              Feed                                                                              T° F.                                                                      Line to                                                                            Pressure,                                                                           Drop Dis.                                                                           Air Rate                                                                           Fiber                                      No.  Tank                                                                              head                                                                              Extruder                                                                           psig  inches                                                                              ml./min.                                                                           O.D.                                       __________________________________________________________________________    86-4 260 200 205  150   16-1/2"                                                                             .191 .037"                                      86-5 260 205 210  150   16-1/2"                                                                             .191 .050"                                      86-7 260 210 215  150   16-1/2"                                                                             .096 .047"                                      86-8 260 210 215  150   23-1/2"                                                                             .096 .044"                                       86-10                                                                             260 215 220  200   31"   .096 .032"                                                              45"   .096 .030"                                      __________________________________________________________________________

                                      Table III                                   __________________________________________________________________________    Extruder Head 2 - Composition 2                                                    T° F.                                                                          T° F.                                                                       Feed Tank                                                   Batch                                                                              Feed                                                                              T° F.                                                                      Line to                                                                            Pressure,                                                                           Drop Dis.                                                                           Air Rate                                                                           Fiber                                      No.  Tank                                                                              Head                                                                              Extruder                                                                           psig  inches                                                                              ml./min.                                                                           O.D.                                       __________________________________________________________________________    98-9 260 200 202  300   16"   .0382                                                                              .035"                                      98-11                                                                              260 200 202  300   23-1/2"                                                                             .0191                                                                              .033"                                      98-12                                                                              260 202 202  350   23-1/2"                                                                             .0191                                                                              .033"                                      98-14                                                                              260 204 204  300   16"   .0191                                                                              .029"                                      98-18                                                                              260 205 205  300   16"   .0096                                                                              .026"                                      98-20                                                                              260 206 206  200   16"   .0096                                                                              .023"                                      98-22                                                                              260 206 206  250   16"   .0076                                                                              .024"                                      __________________________________________________________________________

EXAMPLE 2

Four hollow fiber separation test units were prepared, one from each offiber batches 86-9 and 86-10 (both Composition 1), and two from fiberbatch 98-22 (Composition 2), see Tables I, II and III. The units wereprepared by taking the appropriate dry fiber and potting the ends intoseparate 21/2 × 1/4 inch O.D. stainless steel tubes with an epoxy resin.Potting was done in such a manner to insure that after curing, the endsof the fibers could be exposed by the removal of a small amount of thecured potting resin and fiber. The curing of the potting compound wasdone at room temperature for 24 hours. Each unit was prepared using onefiber with an effective membrane length of 14 inches and an effectivearea of 4.3 cm². All fibers had been crosslinked before potting byheating at 50°-60° C. in an aqueous bath of 3% p-toluene sulfonic acidand 10% sodium sulfate for 90 minutes. The fibers were thoroughly rinsedin distilled water and dried after crosslinking.

Each fiber unit was soaked for three hours in 6M aqueous AgNO₃ toactivate the fiber for an ethylene separation test. After soaking, thefibers were tested individually in a hollow fiber test cell. Each fiberwas supplied with a humidified feed gas mixture of ethane, methane, andethylene under pressure and at the rate of 10 ml./min. In three of thefour units the feed gas was supplied to the inside of the hollow fiber,i.e. the units having fibers 86-9, 86-10 and one of 98-22. In the fourthunit having the other 98-22 fiber the feed gas was supplied to theoutside of the fiber. A purge gas stream of 10 ml./min. of nitrogen wascontinually supplied to the other side of the fiber in each test. Thecomposition of the purge stream exiting the cell was periodicallymonitored to determine the selectivity of the fibers to ethylene andtheir permeability. The results of these tests are summarized in TableIV and they show that the hollow fibers can be used as semi-permeablemembranes for purifying olefins, especially ethylene.

                  Table IV                                                        ______________________________________                                        PERFORMANCE OF HOLLOW FIBERS IN                                               ETHYLENE PURIFICATION                                                                 Feed Gas = 18.37% Methane                                                     39.77% Ethylene                                                               41.86% Ethane                                                                                               Permea-                                        Feed                           tion                                           Pres-                          Rate.**                                 Fiber  sure,   Permeate Composition   ml./cm.sup.2 -                          No.    PSIG    Methane  Ethylene                                                                             Ethane                                                                              S*    min.                               ______________________________________                                        86-9   10      0.06     99.77  0.17  660  0.042                               86-10  10      0.06     99.79  0.15  660  0.026                               98-22                                                                          (feed                                                                        inside)                                                                              15      0.55     95.02  4.43   29  0.00081                             98-22                                                                          (feed                                                                        outside)                                                                             15      0.20     99.55  0.25  335  0.0026                              ______________________________________                                        *S = Selectivity =                                                             ##STR1##                                                                     **Based on log mean diameter of 15.15 mils.                               

Hollow fibers were formed by extrusion essentially in the mannerdescribed in Example 1 using an extrusion mixture having 72 grams of thepolyvinyl alcohol, 128 grams of BCI-819 nylon, 209 ml. of DMSO and 21ml. H₂ O. The extruded fibers were annealed at 290° F. to remove flaws,dried at 75° C. for 2 hours to remove the solvents and cross-linked byimmersion for 60 minutes in a 3% aqueous p-toluene sulfonic acid bath(10% Na₂ SO₄) at 55° F. The cross-linked fibers were dried.

The fibers were assembled in two test units Unit 1 and Unit 2 in themanner described in Example 2 using Dow-Corning Sylgard 184, a siliconepotting resin, to hold the ends of the fibers, there being 4 fibers ineach unit. The assembled fibers had an active length of 11 inches. Thefibers of Unit 1 and Unit 2 were made using an extruder head having a0.030 inch diameter hole and 0.016 inch outside diameter needle. Thefibers themselves had the following cross sections: those in Unit 1 hadan O.D. of 0.020 inch and an I.D. of 0.0055 inch while those in Unit 2had an O.D. of 0.0177 inch and an I.D. of 0.0093 inch. The activesurface area of Unit 1 was about 10 cm² and of Unit 2 about 11.7 cm².The units were stabilized by using them several times to separateethylene from admixture with ethane and methane after impregnation ofthe fibers with an aqueous silver nitrate solution.

Two of these units were made and stabilized as described above. Theunits were then immersed for one hour in 2N aqueous silver nitrate (Unit2) or a 2N silver nitrate 50% glycerol -- 50% water solution (Unit 1),respectively, to impregnate the fibers. The impregnated units weretested for four days in an ethylene separation process in which amethane-ethane-ethylene feed flowed down the center of the fibers at 20psig and at a rate of 10 ml./min. (S.T.P.). A nitrogen purgesupersaturated with water passed over the outside of the fibers atatmospheric pressure and at the rate of 10 ml./min. The nitrogen purgegas was super-saturated with water by bubbling the gas purge through awater trap set at 30° C. The purpose of the purge was to pick up thegases permeating the walls of the fibers. A gas chromatographic analysisof the purge provided both the composition and the permeation rate ofthe gases coming through the fiber walls. During the four day testperiod the unit having no glycerol (Unit 2) performed better than theother unit.

Both of the test units were then impregnated by contact with a 2Naqueous silver nitrate solution containing 0.3% H₂ O₂ for 1 hour at 20psig. This impregnation apparently decreased the glycerol content of thefibers in Unit 1. The units were retested and the operation of Unit 1was considerably better than that of Unit 2 having no glycerol, and thedeactivation rate of Unit 1 was only about one-half of that of Unit 2.The half-life in terms of ethylene permeability for Unit 1 was 12.5 daysand for Unit 2 was 5.5 days. The results of these tests are reported inTable V.

                                      Table V                                     __________________________________________________________________________                              Permeation                                                     Permeate       Rate  S.                                            Unit    Days                                                                             % CH.sub.4                                                                         % C.sub.2 H.sub.4                                                                  % C.sub.2 H.sub.6                                                                  ml/cm.sup.2 min                                                                     (selectivity)                                 __________________________________________________________________________    (Feed      20.7 35.6 43.7)                                                    1.      0  0.32 99.11                                                                              0.57 .033  205                                           (Glycerol)                                                                            1  0.30 99.22                                                                              0.48 .035  234                                            ##STR2##                                                                             2  0.38 98.99                                                                              0.62 .027  180                                                   3  0.43 98.94                                                                              0.63 .026  172                                                   4  0.42 98.85                                                                              0.73 .026  158                                                   7  0.50 98.67                                                                              0.83 .025  137                                                   8  0.57 98.55                                                                              0.88 .022  125                                                   9  0.59 98.06                                                                              1.35 .019  93                                                    10 0.61 98.27                                                                              1.12 .021  105                                                   11 0.71 98.08                                                                              1.21 .016  94                                            __________________________________________________________________________    2.      0  0.42 98.91                                                                              0.67 .028  167                                           (No glycerol)                                                                  ##STR3##                                                                             1  0.48 98.81                                                                              0.71 .025  153                                                   2  0.64 98.66                                                                              0.70 .025  135                                                   3  0.60 98.39                                                                              1.02 .021  112                                                   4  0.63 98.25                                                                              1.13 .021  103                                                   7  1.09 97.03                                                                              1.88 .012  60                                                    8  1.50 95.76                                                                              2.74 .012  42                                                    9  1.57 95.83                                                                              2.61 .010  42                                                    10 1.71 95.23                                                                              3.07  .0096                                                                              37                                                    11 1.89 95.24                                                                              2.88  .0090                                                                              37                                            __________________________________________________________________________     These data show that the presence of glycerol increased the permeation        rate and selectivity of the separation.                                  

EXAMPLE 4

A mixture containing 75 gms. of the polyvinyl alcohol described inExample 1, 175 gms. of nylon (Belding BCI-819), 250 ml. of dimethylsulfoxide and 25 ml. of distilled water was stirred for 2 hours at 270°F. in the feed tank of the extruder of Example 1. The extruder wassealed, and the temperature of the extruder tank was set at 250° F. Theline between the tank and the extruder head was at 206° F., and theextruder head was at 225° F. The extruder head had an annular openingwith the O.D. being 0.050 inch and the I.D. being a hypodermic needle0.025 inch in outside diameter. During extrusion, air was pumped throughthe hypodermic needle to maintain the fiber bore open.

Fibers were extruded at the rate of 4'/min. when the tank waspressurized with 250 psig of nitrogen, and air was pumped through thehypodermic needle at the rate of 0.061 ml./min. The fiber was allowed todrop 11 inches to stretch it somewhat. After the fiber cooled (at least30 sec.) it was pulled at a rate of 3.7 feet per minute through a 3-footlong heated furnace maintained at 252° F. This time-temperaturecombination was sufficient to fuse the polymer particles withoutdistorting the fiber shape. The fiber is under some small stress at thispoint and does stretch. The final fiber had an O.D. of 0.028 inch. Afterthis melting step, the fibers are fairly elastic and can be stretchedfurther. When stretching was done to a similarly prepared fiber, theresulting fiber had a 0.020 inch O.D.

EXAMPLE 5

A hollow fiber A was extruded essentially in accordance with theprocedure described in Example 1 using a mixture containing 65 weight %of BCI-819 nylon and 35 weight % of the polyvinyl alcohol of Example 1to which was added 95 weight % of DMSO and 15 weight % H₂ O based on thetotal weight of the nylon and polyvinyl alcohol. The fiber was thencrosslinked by immersion for 1 hour in a 50° C. aqueous bath containing10% sodium sulfate and 3% p-toluene sulfonic acid. The crosslinked fiberwas then washed thoroughly with distilled water. The fiber was swollenby being allowed to stand overnight in a humidified atmosphere beforestretching.

Stretching of the fiber was accomplished by feeding it in a waterswollen state to a spool 1.5 in. diameter, which in turn fed the fiberto another spool 3.0 in. diameter, which in turn supplied the fiber to aspool 6.0 in diameter. From the last spool the fiber was taken up on15/8 inch diameter spool. The first three spools were affixed on thesame shaft and thus were maintained at the same angular velocity. Thetakeup spool was operated at a rate 5 times that of the third spool. Aconstant stress of 62.5 grams was applied to the fiber before it reachedthe first spool. Table VI summarizes the diameter changes found forfiber A after stretching.

A second experiment was carried out with a hollow fiber (Fiber B) whichwas of a composition similar to fiber A and was extruded, annealed andcrosslinked in the same manner as fiber A. Fiber B was stored in waterfor at least one hour to produce a swollen condition. A 10-inch sectionof fiber B was then stretched by hand pulling until the relaxed lengthof the fiber was 20 inches. The stretching results are also summarizedin Table VI.

                  Table VI.                                                       ______________________________________                                        Stretching of Hollow Fibers -                                                 Change in Diameters.                                                                     Outside Diameter                                                                          Inside Diameter                                                   (mils)      (mils)                                                 ______________________________________                                        Fiber A,                                                                              initial  24            --                                                     final    15            --                                             Fiber B,                                                                              initial  36            12                                                     final    24             8                                             ______________________________________                                    

Fiber B, both unstretched and stretched, was evaluated as a membrane forseparating ethylene in a test cell in a manner similar to the proceduredescribed in Example 2. The fiber was secured in the cell by pottingwith a silicone potting compound (Slygard 184, Dow-Corning Corp.). Thecell was filled with 6 M aqueous AgNO₃ and allowed to soak for 60minutes. This allows AgNO₃ to enter the fiber. The excess AgNO₃ was thenremoved from the cell. A hydrocarbon feed gas was then supplied to theinside of the hollow fiber at 10 psig and 10 ml./min. The outside of thehollow fiber was continually purged with a humidified 10 ml./min. streamof helium. Sampling of the purge stream exiting the cell was doneperiodically. The samples were analyzed by gas chromatography. Fromthese analyses the permeation rate for and selectivity to ethylene ofthe fibers were determined. The results of these tests are summarized inTable VII.

                                      Table VII                                   __________________________________________________________________________    PERFORMANCE OF FIBER B AS AN ETHYLENE SELECTIVE MEMBRANE                                   Feed Gas Composition: methane 18.6 wt %                                       Ethylene 39.7 wt %                                                            Ethane 41.7 wt %                                                 Fiber, Time Area*                                                                             Permeate Comp., Wt. % (He-free)                                                                    Permeation Rate                          After Startup                                                                             (Cm.sup.2)                                                                        Methane                                                                             Ethylene                                                                            Ethane                                                                              S**                                                                              (ml/cm.sup.2 -min)                       __________________________________________________________________________    Fiber B,                                                                           stretched                                                                            4.6                                                                    2 hrs.     0.04  99.82 0.14  843                                                                              0.041                                         6 hrs.     0.04  99.82 0.14  842                                                                              0.045                                    Fiber B,                                                                           unstretched                                                                          5.35                                                                   2 hrs.     0.07  99.80 0.13  750                                                                              0.013                                         6 hrs.     0.08  99.77 0.16  666                                                                              0.018                                    __________________________________________________________________________     *A = surface area of fiber utilized for the separation                        **S = Selectivity, see Table IV                                          

It is evident from Table VI that stressing of the cold water swollenfiber produces thinner fibers. Table VII shows that these fibers areboth more selective and more permeable to ethylene than unstretchedfibers.

EXAMPLE 6

The following compositions were heated at 260° F. for 2 hours withvigorous stirring of the melt:

1. 192 gms. BCI-819 nylon 108 gms. PVA of Example 1 300 mls. DMSO 30 ml.Distilled H₂ O

2. 192 gms. BCI-819 nylon 108 gms. PVA of Example 1 330 mls. DMSO

Fibers were made from these compositions in a manner similar to thatdescribed in Example 1. The fibers made from composition (1) were whiteas were the separate polymer powders used; however, the fibers made fromcomposition (2) were yellow to brown and exhibited undesirabledegradation. The presence of the water in composition (1) wasresponsible for the improved results.

EXAMPLE 7

Fibers have been extruded essentially by the method described in ExampleI. 180 Gms of BCI-819 nylon and 120 gms of the PVA of Example I weretumbled together in a Renco evaporator for 20 minutes. To this mixturewas added 500 ml. of a DMSO-H₂ O mixture (500 ml. DMSO to 25 ml. H₂ O).The mixture was allowed to gel at room temperature for at least 20hours. The mix was then put into the extruder tank and melted withstirring at 250° F. for at least two hours. The tank was sealed andnitrogen pressure (400 psig) forced the melt into a Zenith Laboratorymetering pump. The pump drives the melt at a controlled rate through a50-micron stainless steel filter and out of the extruder head. The headcontains an annular die having a hole 0.040 inch in diameter with a0.016 inch O.D. tubing in the center of the hole, to form the fiber. Asyringe pump meters air through the center tubing in the die. The centertubing is a hollow, stainless steel tubing. The air keeps the fiber fromcollapsing and controls the O.D.-to-I.D. ratio of the fiber. Typicalvalues for the temperature and feed rates for making the fiber are asfollows:

T_(tank) = 250° F.

T_(line) A = 250° F.

T_(pump) = 250° F.

T_(line) B = 195° F.

T_(head) = 170° F.

Polymer rate = 2.0 ml/min

Air rate = 0.076 ml/min

The fibers, once extruded, are stretched by falling from 3 inches to 30inches to an isopropanol quench bath at 0° C. Fibers are extruded andcollected for 30 min and are then soaked an additional 30 min in the 0°C. bath. They are next dried at 75° C. for 2 hours, crosslinked in anaqueous bath (3% p-toluene sulfonic acid plus 10% Na₂ SO₄) at 55° C. for1 hour, washed three times in distilled water, and air-dried for atleast 24 hours.

It is claimed:
 1. A composition consisting essentially of hydrophilicpolymer and di(lower alkyl) sulfoxide solvent, said polymer consistingessentially of film-forming N-alkoxyalkyl nylon and sufficientwater-soluble polyvinyl alcohol to enhance the hydrophilic properties ofthe nylon, said composition having a sufficient amount of water toinhibit degradation of the polymer at elevated temperatures suitable forextrusion of the composition.
 2. The composition of claim 1 in which thedi(lower alkyl) sulfoxide is dimethyl sulfoxide.
 3. The composition ofclaim 2 in which the polymer has about 30 to 85 weight % of said nylonand about 15 to 70 weight % polyvinyl alcohol based on their total. 4.The composition of claim 3 in which the dimethyl sulfoxide is about 70to 400 weight % based on the total polyamide and polyvinyl alcohol. 5.The composition of claim 1 in which the nylon is an N-methoxymethylnylon.
 6. The composition of claim 5 in which the polymer has about 40to 70 weight % of said nylon and about 30 to 60 weight % polyvinylalcohol based on their total.
 7. The composition of claim 6 in which thedi(lower alkyl) sulfoxide is about 90 to 250 weight % based on the totalnylon and polyvinyl alcohol.
 8. The composition of claim 7 in which thedi(lower alkyl) sulfoxide is dimethyl sulfoxide.
 9. The composition ofclaim 8 in which the water is about 1 to 30 weight % based on the totalweight of the dimethyl sulfoxide and water.
 10. An extrusiblecomposition consisting essentially of polymer consisting essentially ofabout 30 to 85 weight % of N-alkoxyalkyl nylon and about 15 to 70 weight% of water-soluble polyvinyl alcohol based on their total, about 70 to400 weight % of di(lower alkyl) sulfoxide based on the total weight ofthe nylon and polyvinyl alcohol, and about 1 to 30 weight % of waterbased on the total weight of the water and di(lower alkyl) sulfoxide.11. The composition of claim 10 in which the polymer has about 40 to 70weight % nylon and about 30 to 60 weight % polyvinyl alcohol based ontheir total.
 12. The composition of claim 11 in which there is about 90to 250 weight % di(lower alkyl) sulfoxide based on the total weight ofthe nylon and polyvinyl alcohol, and about 2 to 20 weight % water basedon the total weight of the water and di(lower alkyl) sulfoxide.
 13. Thecomposition of claim 12 in which the nylon is an N-methoxymethyl nylon.14. The composition of claim 13 in which the di(lower alkyl) sulfoxideis dimethyl sulfoxide.
 15. A composition consisting essentially ofhydrophilic polymer and di(lower alkyl) sulfoxide solvent, said polymerconsisting essentially of film-forming N-alkoxyalkyl nylon having about5 to 60% of the amide groups substituted with alkoxyalkyl groups andsufficient water-soluble polyvinyl alcohol to enhance the hydrophilicproperties of the nylon, said composition having a sufficient amount ofwater to inhibit degradation of the polymer at elevated temperaturessuitable for extrusion of the composition.